maint: DECIMAL_DIGIT_ACCUMULATE uses stdckdint.h

maint: DECIMAL_DIGIT_ACCUMULATE uses stdckdint.h

* src/system.h: Include <stdckdint.h>, since the new
DECIMAL_DIGIT_ACCUMULATE uses it.
Do not include stdckdint.h from files that also include system.h.
(DECIMAL_DIGIT_ACCUMULATE): Omit last arg, which is no longer needed.
Reimplement by using C23-style stdckdint.h’s ckd_mul and ckd_add,
as that’s more standard and is more likely to generate better code.